Vegetable-derived oils have gradually replaced animal-derived oils and fats as the major source of dietary fat intake. However, saturated fat intake in most industrialized nations has remained at about 15% to 20% of total caloric consumption. In efforts to promote healthier lifestyles, the United States Department of Agriculture (USDA) has recently recommended that saturated fats make up less than 10% of daily caloric intake. To facilitate consumer awareness, current labeling guidelines issued by the USDA now require total saturated fatty acid levels be less than 1.0 g per 14 g serving to receive the “low-sat” label and less than 0.5 g per 14 g serving to receive the “no-sat” label. This means that the saturated fatty acid content of plant oils needs to be less than 7% and 3.5% to receive the “low-sat” or “no-sat” label, respectively. Since issuance of these guidelines, there has been a surge in consumer demand for “low-sat” and “no-sat” oils. To date, this demand has been met principally with canola oil, and to a much lesser degree with sunflower and safflower oils.
While unsaturated fats (monounsaturated and polyunsaturated) are beneficial (especially when consumed in moderation), saturated and trans fats are not. Saturated fat and trans fat raise undesirable LDL cholesterol levels in the blood. Dietary cholesterol also raises LDL cholesterol and may contribute to heart disease even without raising LDL. Therefore, it is advisable to choose foods low in saturated fat, trans fat, and cholesterol as part of a healthful diet.
The characteristics of oils, whether of plant or animal origin, are determined predominately by the number of carbon and hydrogen atoms in the oil molecule, as well as the number and position of double bonds comprised in the fatty acid chain. Most oils derived from plants are composed of varying amounts of palmitic (16:0), stearic (18:0), oleic (18:1), linoleic (18:2) and linolenic (18:3) fatty acids. Conventionally, palmitic and stearic acids are designated as “saturated,” because their carbon chains are saturated with hydrogen atoms, and hence have no double bonds; they contain the maximal number of hydrogen atoms possible. However, oleic, linoleic, and linolenic acids are 18-carbon fatty acid chains having one, two, and three double bonds, respectively, therein. Oleic acid is typically considered a monounsaturated fatty acid, whereas linoleic and linolenic are considered to be polyunsaturated fatty acids. The U.S.D.A. definition of “no sat” oil products, meaning those having less than 3.5% saturated fatty acid content, is calculated as the combined saturated fatty acid content by weight (as compared to the total amount of fatty acids).
Canola oil has the lowest level of saturated fatty acids of all vegetable oils. “Canola” refers to rapeseed (Brassica) which has an erucic acid (C22:1) content of at most 2% by weight, based on the total fatty acid content of a seed (preferably at most 0.5% by weight, and most preferably essentially 0% by weight), and which produces, after crushing, an air-dried meal containing less than 30 μmol/g of glucosinolates in defatted (oil-free) meal. These types of rapeseed are distinguished by their edibility in comparison to more traditional varieties of the species.
It is postulated that, in oilseeds, fatty acid synthesis occurs primarily in the plastid. The major product of fatty acid synthesis is palmitate (16:0), which appears to be efficiently elongated to stearate (18:0). While still in the plastid, the saturated fatty acids may then be desaturated by an enzyme known as acyl-ACP delta-9 desaturase, to introduce one or more carbon-carbon double bonds. Specifically, stearate may be rapidly desaturated by a plastidial delta-9 desaturase enzyme to yield oleate (18:1). In fact, palmitate may also be desaturated to palmitoleate (16:1) by the plastidial delta-9 desaturase, but this fatty acid appears in only trace quantities (0-0.2%) in most vegetable oils. Thus, the major products of fatty acid synthesis in the plastid are palmitate, stearate, and oleate. In most oils, oleate is the major fatty acid synthesized, as the saturated fatty acids are present in much lower proportions.
Newly-synthesized fatty acids are exported from the plastid to the cytoplasm. Subsequent desaturation of plant fatty acids in the cytoplasm appears to be limited to oleate, which may be desaturated to linoleate (18:2) and linolenate (18:3) by microsomal desaturases acting on oleoyl or lineoleoyl substrates esterified to phosphatidyl choline (PC). In addition, depending on the plant, oleate may be further modified by elongation (to 20:1, 22:1, and/or 24:1), or by the addition of functional groups. These fatty acids, along with the saturated fatty acids, palmitate and stearate, are then assembled into triglycerides in endoreticular membranes.
The plant acyl-ACP delta-9 desaturase enzyme is soluble. It is located in the plastid stroma, and uses newly-synthesized fatty acids esterified to ACP, predominantly stearyl-ACP, as substrates. This is in contrast to the other delta-9 desaturase enzymes, which are located in the endoplasmic reticular membrane (ER, or microsomal), use fatty acids esterified to Co-A as substrates, and desaturate both the saturated fatty acids, palmitate and stearate. U.S. Pat. Nos. 5,723,595 and 6,706,950 relate to a plant desaturase.
The yeast delta-9 desaturase gene has been isolated from Saccharomyces cerevisiae, cloned, and sequenced. Stukey et al. (1989) J. Biol. Chem. 264:16537-44; Stukey et al. (1990) J. Biol. Chem. 265:20144-9. This yeast gene has been introduced into tobacco leaf tissue (Polashcok et al. (1991) FASEB J. 5:A1157; Polashok et al. (1992) Plant Physiol. 100:894-901), and was apparently expressed in this tissue. Further, this yeast gene was expressed in tomato. See Wang et al. (1996) J. Agric. Food Chem. 44:3399-402; and Wang et al. (2001) Phytochemistry 58:227-32. While some increases in certain unsaturated fatty acids, and some decreases in certain saturated fatty acids, were reported for both tobacco and tomato using this yeast delta-9 desaturase gene, tobacco and tomato are clearly not oil crops. This yeast gene was also introduced into Brassica napus. U.S. Pat. No. 5,777,201.
A different fungal acyl-CoA delta-9 desaturase from Aspergillus nidulans has been introduced into canola, thereby achieving reduced saturated fatty acid levels in seed oil. U.S. Patent Application Publication US 2008/0260933 A1. The A. nidulans acyl-CoA delta-9 desaturase provided greater depletion of stearate (61-90%) than the more abundant palmitate fatty acids (36-49%) in the seed oil.